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close this book Soil conservation techniques for hillside farms
close this folder Soil conservation strategies
close this folder Strategies in cultivation systems characterized by extensive soil disturbance
View the document Crop rotation
View the document Contour barriers (live, dead and mixed barriers)
View the document Contour ditches (drainage and infiltration ditches)
View the document Terraces (individual, discontinuous narrow, and continuous bench terraces)
View the document Waterways from draining excess water for fields
View the document Gully prevention and control

Strategies in cultivation systems characterized by extensive soil disturbance

CONSERVATION STRATEGIES AIMED AT MINIMIZING SOIL LOSS AND INCREASING WATER INFILTRATION IN CULTIVATION SYSTEMS WHERE THERE IS EXTENSIVE SOIL DISTURBACE

Even though the techniques described in the previous section are more effective in reducing the environmental damages associated with man's agricultural activity, the techniques described in this section are much easier to promote because they are much easier to integrate into the traditional cultivation systems.

Crop rotation

The use of a crop rotation sequence, rather than continual successive plantings of the same crop can play a role in reducing soil erosion as well as provide other benefits (vary the rooting depth and thereby moisture and nutrient uptake, restore soil structure, break pest and disease cycles, help maintain soil fertility, Hudson 1981). When land is used repeatedly for cultivating relatively open row crops (i.e. corn, beans, tobacco, etc.), a rotation with a denser green manure or forage crop can reduce erosion because of the ground cover provided. Also the maintenance of soil fertility and structure will allow the next planting of the more open crop to grow more vigorously and develop better root systems, thereby helping reduce some soil loss. Generally, however, on sloping lands (5-10% +), crop rotations alone will have little effect on erosion, and should be used in combination with other techniques.

Contour barriers (live, dead and mixed barriers)

Contour barriers are contour strips which intercept downslope flowing water and soil particles. These barriers slow down the water movement and reduce its erosive force. They also filter out and trap many of the suspended soil particles, keeping them from being washed out of the field. A long term advantage of barriers is that soil tends to build up behind them, creating a terrace effect. Barriers can be classified as live (strips of living plants), dead (rocks, crop residues), or mixed (a combination of the previous two).

Live barriers are strips of vegetation planted along the contour which serve to anchor the soil in place with their roots and to slow down the movement of water downslope with their stems and leaves (Fig. 8). They are planted above hillside ditches to prevent them from filling with soil and also by themselves to prevent hillside erosion. me most common types of live barriers are plants of the grass family because of the dense foliage and root nets produced. As soil builds up behind them, a bench terrace with a grass protected riser (sloping bank) is formed (see section on bench terraces). In addition the grasses are valuable as forage for animals, or for human consumption in the case of sugar cane or lemmon grass. Many species of plants have great potential as useful live barriers, some possibilities are listed in Table 1. The table is by no means all inclusive, the possibilities are almost limitless, especially when considering agroforestry systems where contour strips of nitrogen-fixing and/or wood or fruit producing trees may be used as barriers. An effort should be made to discuss the available barrier plants in the area with farmers so that each one can select the ones most suited to their needs.

As a management practice it is advisable to cut forage barrier plants and carry them to livestock rather than letting livestock graze them. If the barriers are not well established, the animals may uproot or overgraze them, thus wiping out the barrier. If farmers use the practice of opening their fields to grazing after the harvest, then an unpalatable barrier or planting early in the year to ensure a well established barrier is necessary. Maintenance of the live barriers is extremely important. Open spaces should be replanted so that the barrier forms an effective soil filter. Barriers should be cut or pruned to avoid excessive shading or root competition with the crops.


TABLE 1 Some suggested live barrier plants


TABI.E 1 (Continued)


Fig. 8. Contour live barriers

In planting contour barriers, the spacing from one barrier to the next depends on the slope of the land, with barriers spaced closer together on steep slopes and farther apart on gentler slopes. (Table 2)

TABLE 2

SPACING OF CONTOUR BARRIERS ACCORDING TO THE SLOPE

SLOPE (%)

ANNUAL CROPS Distance (m)

PERRENIAL CROPS Distance (m)

5

20.0

25.0

10

15.1

20.1

15

10.1

18.2

20

9.2

15.3

25

8.2

15.5

30

6.8

12.5

35

6.4

12.7

40

6.5

9.7

60

 

7.0

From Suarez Castro 1980, modified to show distance along ground surface rather than horizontal distance.

Contour planting of a crop and contour strip cropping are also techniques which may be considered functionally as live barriers since they are contour plantings which serve to control hillside erosion. The planting of windbreaks is also a use of live barriers (trees or tall grasses) in this case to avoid erosion or crop damage due to wind rather than water.

Dead barriers function similar to live barriers, the difference being that they are composed of rocks, plant residues, or other non-living materials. (Fig. 9). If rocks are present in a field, it is useful to construct these and in the process make the soil easier to work. Another advantage is that they can be completed during the dry season, meaning that they are in place and functioning at the start of the rainy season. If enough rocks are present' the barriers can be constructed as rock walls of sufficient height so that bench terraces are famed as the soil fills in behind each wall. If sufficient rocks are not present, the barriers lose their effectiveness as the soil fills in behind them, and they should be supplemented with the planting of live barriers.


Fig. 9. Contour dead barriers

The construction of dead barriers is simple, but requires a lot of manual labor. Once contour lines are marked out according to the spacings giving in Table 2,a hoe is used to form a furrow which serves to anchor the barrier (~ 20 cm deep). men the materials are laid out to form walls along the contour lines. In some cases, mixed barriers are used, a combination of live and non-living materials. This can consist of strips of trees with the intervening spaces filled with rocks or crop residues. (Fig. 8), or combinations of grass and rock barriers. If using plant residues, be aware that as they decompose they lose their effectiveness as barriers and will erode away, needing to be supplemented.

Contour furrows or raised planting beds may also be considered functionally as contour barriers. (Fig. 10). These techniques are useful in the cultivation of vegetables, basic grains, or any raw crops in which the soil is to be intensively cultivated. The contour furrows or beds serve various purposes: reducing water movement and therefore soil erosion down the slope, permitting drainage of excess soil moisture from the planting bed, providing for a more even distribution of irrigation or rainwater, and avoiding compaction of the planting bed surface by providing walkways The construction is simple. The furrows are plowed if possible, along measured contour lines, then cleaned out with a hoe or shovel, spreading the soil in the space between the furrows. The width between furrows is variable and depends on the crop and any irrigation requirements.


Fig. 10. Contour planting beds

Contour ditches (drainage and infiltration ditches)

Contour ditches serve many of the same purposes as contour barriers, in addition they completely stop dawnslope water movement as the water falls into the ditch. These structures are some of the mast useful for small-scale hillside farming singe they require less work than terraces, are simple to build, and can be used to either divert or to retain water. If constructed at a 1% slope* they divert excess water to protected drainageways, reducing soil erosion and leaching of nutrients. (Fig. 11). The uppermost ditch, called stormwater drain by Hudson 1981, is very important if a great deal of water enters from above the field.


Fig. 11. Contour drainage ditch

*Hudson (1981) recommends a 0.25% slope for contour ditches (channel terrapin). Due to the relative inaccuracy of the rustic levels often used (See Apendix 4), a 1% slope is recommended here to insure water flow in the proper direction.

If it is desirable to retain as much water as possible, earthen dikes can be left in the ditches or the ditches can be constructed at a 0% slope so that water infiltrates into the soil and is not diverted outside the field. (Fig. 12)


Fig. 12. Contour infiltration ditch

The ditches are constructed using pickaxes and shovels. If possible, plowing the contour line makes the construction much easier. The first stage in the construction is to excavate a 12 inch wide by 12 inch deep ditch (ditches can be constructed of any size, if desired). Then the banks are formed by cutting a slanted wall at each side. The removed earth is placed in a mound 6 - 9 inches below the lower lip of the ditch. A live barrier is necessary above the upper edge to prevent filling with soil. Especially on steeper slopes, it is often advantageous to plant the live barrier first, several months or one season in advance, so that the ditch will be adequately protected once built.

Table 3 should be consulted for the appropriate distance between ditches on hillsides of a given slope. If the ditches are to be dug with a 1% slope to drain excess water, the 1 or 2 meters before emptying into a protected drainageway should have a slightly steeper slope, (1-2%) to facilitate drainage. Care should also be taken not to drain excess water into neighboring fields, houses, or other areas which may result in problems. (See "Waterways for draining excess water from fields").


TABLE 3 Spacing of contour hillside ditches

Terraces (individual, discontinuous narrow, and continuous bench terraces)

Terraces serve the functions of stopping downslope soil and water movement and also give the advantage of providing a flat surface for the planting of crops, thereby further reducing the possibility of erosion.

Some important points should be noted before deciding to build terraces. Since the formation of terraces requires a maximum of soil disturbance and rearrangement, they are extremely susceptible to erosion if not properly measured, compacted, and maintained with risers(sloping banks) protected by vegetation. Also when considering terracing a steep slope (25-30% +) it should be noted that a much deeper cut and fill will be needed, much more land space will be lost in the sloping terrace walls, and the useful planting space is likely to be extremely narrow unless a very deep soil is present.

Individual terraces are constructed to provide a level platform for the growth of an individual tree. They are always used in combination with another type of conservation structure, such as contour ditches, since the small platforms by themselves do not appreciably control surface water movement. (Fig. 13). When planting a hillside with individual terraces, the layout should be in an equilateral triangular or hexagonal pattern, with the distances varying according to the variety of tree being planted. The terraces generally have a diameter of 1 to 1.5 meters and should be throughly compacted to prevent collapsing.


Fig. 13. Individual terraces.

Discontinouous narrow terraces (orchard berraces/provide both -a flat platform for planting crops and an inverse slope which allows it to serve as a drainage or infiltration ditch.(Figs. 14, 15).


Fig. 14. Discontinuous narrow terrace (orchard terrace)


Fig. 15. Discontinuous narrow terrace (X.S.)

 

Construction is done, after plowing if possible, with pickaxes and rakes or hoes Some design information is given in Table 4. Construction is most rapid in soft, deep soils.

In these cases the earth from the cut section is merely moved to the fill section, compacted, and raked smooth. If it is judged necessary to remove the topsoil and redistribute it later, then a construction sequence such as sequence "B" given for the bench terraces is recommended (Fig. 18). In order to keep the risers from eroding, grass should be planted on them. The drainage way for this type of structure should be constructed similar to that of the hillside ditch, slightly steeper right before emptying into the protected drainage area. (See 'Waterways for draining excess water from fields").

TABLE 4 DISCONTINUOUS NARROW

TERRACE CONSTRUCTION GUIDE

slope (%)

DISTANCE BETWEEN CANALS (meters)

TOTAL CANAL WIDTH(Platform Plus Walls) (meters)

5

18

2.22

10

14

2.32

15

13

2.40

20

12

2.52

30

12

2.71

40

12

3.00

From Michaelsen, 1980

*Based on 2m platform width, 1.5 wide platforms are also used, especially on steeper slopes with thin soils.

Continuous bench terraces are staircase-like structures which diminish erosion because the reshaping of the Land surface results in the planting of all crops on gently inversely sloping platforms (Fig. 16). The mast feasible application is in intensively worked vegetable plots where each planting bed may be a separate terrace.

Bench terraces can be designed level (0% slope) for water retention or with a slight slope (0-1%) to facilitate water drainage or distribution of irrigation water. (See "Waterways for Draining Excess Water From Fields']


Fig. 16. Continuous bench terraces (X.S.)

There are many different terrace design specifications published in soil conservation books, but only the basic features are described here. Table 5 may be used as a general guide for designing terraces with the total terrace width dependent upon the slope of the hillside and the depth of fertile or tillable soil.

TABLE 5 BENCH TERRACE CONSTRUCTION GUIDE

SLOPE (%)

SOIL DEPTH* (Meters)

TOTAL TERRACE WIDTH (Meters)

PLATFORM WIDTH (Meters)

 

.2

1.68

1.30

 

.3

2.5

1.94

20

.4

3.34

2.60

 

.5

4.26

3.30

 

.6

5.02

3.90

 

.2

1.16

.80

 

.3

1.72

1.20

30

.4

2.3

1.60

 

.5

2.98

2.10

 

.6

3.46

2.40

 

.2

.90

.56

 

.3

1.32

.82

40

.4

1.78

1.10

 

.5

2.25

1.40

 

.6

2.68

1.66

 

.2

.74

.40

 

.3

1.10

.60

50

.4

1.46

.80

 

.5

1.84

1.00

 

.6

2.20

1.20

*"Depth of A Horizon" in original changed to permit use in eroded areas where horizons are often indistinct.

From Suarez Castro, 1980

Bench terraces are constructed using pickaxes, hoes, and rakes. In some areas, large, specially designed hoes are available. Construction is much easier if the section of earth to be removed (cut section) is plowed beforehand to loosen the soil. If construction is being undertaken during the rainy season, it is advisable to begin construction near the drainage area and with the uppermost terrace. In this way, any rainwater will drain off without damaging the terraces.

There are several ideas as to the best method of constructing a series of terraces. One, which may require several years for completion, is the planting of a live barrier grass or the construction of rock walls along the contour. Over time terraces are formed as soil fills in behind them. Two other construction sequences are presented here. (Figs. 17,18)


Fig. 17. Bench terrace construction sequence "A"


Fig. 18. Bench terrace construction sequence ''B"


Fig. 18. Bench terrace construction sequence "B"

Waterways from draining excess water for fields

In order to avoid problems of erosion at the site of emptying and to reduce the speed of watershed runoff following rains; soil conservation structures should be designed for water retention and infiltration whenever possible. If it is judged necessary to drain water from a field, special care should be taken in selecting areas in which to deposit all the diverted drainage water.

Possible drainage areas include pasture areas with a thick ground cover, orchards, or forested areas; where infiltration of the diverted water can probably occur with a minimum of erosion, especially if the water is spread over a large area. Existing waterways may also be used as drainage sites, although one should avoid exaggerating erosion problems by diverting water into areas of active gully formation. In all of these cases, if erosion problems are noted upon diverting runoff water, then a permanent, protected site for receiving runoff should be designed and constructed as soon as possible.

Protected drainageways can be formed by reshaping natural drainageways or digging artificial drainageways of a low, broad shape, protecting them from erosion by lining with rocks, planting grass, and/or placing drop structures or check dams periodically. (Fig. 19).


Fig. 19. Water drainageway protected against erosion by rock lining.

Waterway design specifications are given in more complete soil conservation guides (Hudson, 1981; Suarez Castro, 1980), allowing the construction of waterways of an adequate size and the selection of an adequate lining method based on local metereological conditions, soil properties, the area of land involved land slope, and the type of protecting lining to be used. If sufficient data is available for the work area, such information can be useful in designing a drainageway of sufficient capacity, without overdesigning it and involving excess work. If however, insufficient local data is available, or it is desired to teach farmers the conservation techniques without discouraging them with complicated tables or formulas, then the appropriate size can be estimated. Hudson (1981) mentions an extremely simplified method ignoring local climatic or edaphic factors: construction of a shallow (30 cm) drainageway measuring one meter wide for every hectare of land area in the drainage basin. While this is probably oversimplified, it does allow one to design drainages of probably adequate dimensions without having to deal with complicated methods.

When no appropriate drainage area is available around a field, then retention wells can be dug and water diverted into, and stored in them while it gradually enters the soil or evaporates. (Fig. 20)


Fig. 20. Retention well as site for diverting runoff water.

Special care should be taken to have the waterways and retention wells completely constructed and well vegetated, if grass lined, before diverting drainage water into them. This can be done by preparing them a season or two ahead of time or by preparing a temporary drainage site until the permanent one is ready.

Gully prevention and control

Gullies form whenever enough water flows in a concentrated area to remove the protecting vegetation and stars digging in and carrying topsoil and eventually subsoil. Some gullies form naturally, others are the result of man's activity, such as redirecting water drainage patterns to protect roads, buildings, or fields. If not controlled, gullies tend to enlarge bath in length and width. Both the soil destabilization and the large amounts of water running through them can eventually ruin fields, roads, or buildings located nearby. When using contour ditching or terracing techniques described in this guide, or carrying out other construction activities requiring the altering of drainage patterns, gully formation can result if the drainageway is not properly protected with rock or grass lining, with retention wells, or with a well vegetated area capable of absorbing the water safely (see previous section an "waterways for draining excess water from fields").

Whatever the cause of gully formation, control measures should be undertaken as soon as possible to prevent further expansion, more damages, and an even more costly control due to the larger size in the future. The most effective control technique is to capture and redirect the source of water responsible. This can be done by means of a gently sloped (1%) contour ditch (Fig. 21). This is appropriate only when a suitable adjacent area is available for drainage or if retention wells can be dug to receive the water. Sometimes large livestock watering tanks are filled with water diverted from gullies.


Fig. 21. Contour diversion ditch and check dams for use in gully control

Once water no longer enters the gully from above, the soil is often stabilized enough so that revegetation and filling in occur naturally without any further care. The gully, should however be protected from further damage due to cattle trampling and overgrazing, or cultivation.

When it is not appropriate to divert the incoming water by use of a contour ditch; if the gully soils, steep surfaces or climate do not permit natural recovery; or if a mere rapid revegetation and refilling is desired, then check dams can be placed across the gully at frequent intervals (Fig. 21). These can be made of rocks, wood, wire mesh, etc., and serve to reduce the velocity of any water still flowing and to trap soil particles helping to stabilize the gully so that revegetation can occur. The gully can also be more rapidly revegetated by planting trees, shrubs, or grasses, rather than waiting for natural colonization to take place.